|Cody Wilson - He wants to kill you|
Now, we're not quite all the way there yet. While any high-end 3D printer is capable of producing a device with the complexity and tolerances of a gun, the materials used are typically plastics and light ceramics that are not nearly robust enough to survive the heat and pressure associated with modern ammunition. The record for service life, as far as anyone knows, is a .223 rifle Cody produced that fired six rounds before becoming unusable. Even so, the writing is on the wall. It's not a technology problem any more, merely one of material science, and that happens to be one of the areas Americans have historically excelled in.
So the interesting question is not if this is possible or how it works or how much it costs. Like aerial robotic murder and cyber-destruction of real physical installations and intrusive remote surveillance, this is another example of an evolutionary technology that has profound implications on the world of the near and medium-term future. What will a world be like where guns are instantly available as a single-use disposable item, when anybody can produce a deadly weapon without any human intervention or regulation, when any dispute can become a gunfight with one mouse click? Nations with restrictive firearms laws will have no choice but try to regulate ammunition, and nations with liberal firearms laws will find those policies resulting in yet another dimension of madness and bloodshed.
If I may get slightly technical here, consider the now infamous AR-15. The AR is comprised of two main parts groups called the upper and lower receiver groups. The upper receiver group contains the barrel and is not considered a firearm, so it is not legally controlled. Anyone can order them over the Internet and own them. The "gun" is the lower receiver, which contains the action, trigger group and chamber. This is worth mentioning because it may be much easier to print a lower receiver only, and then it is a simple matter of mating it to an upper with the regular steel barrel. There is, of course, no reason why that downloadable lower receiver can't have a selector for full auto fire - that's nothing but a different mechanical arrangement.
I don't know about you, but I look into the future I see a place of violence and death. Just as ethnic, sectarian, tribal and class hatreds become the primary drivers of human interaction and greed and inequality leads inevitably to hunger and hopelessness, we have a series of powerful, deadly technologies becoming available to anyone, many of them inexpensive and simple to produce. With intelligent, autonomous drones bringing sudden death from the skies and printable guns supplying neighborhood-level uprisings, it's very hard to imagine how one might ever feel safe in this world we are creating.